Various forms of herbal medicine polysaccharides have become valuable as health supplements worldwide (1, 2), suggesting that administration of such polysaccharides may improve innate immunity in vivo. The underlying molecular mechanisms, however, still remain ambiguous.
To design therapy against cancer, it is desirable to seek molecular targets of cancer or cancer stem cells that are absent from normal cells. Aberrant glycosylation is often associated with tumor progression and was first described by Meezan et al. in 1969 with the demonstration that cancer glycans differ from healthy cells. (Meezan E, et al. (1969) Biochemistry 8:2518-2524.) Aberrant terminal fucosylation as well as sialylation in tumor-associated glycans is one of several glycosylation events important in cancer progression (3, 4), and such unusual glycans have recently been used for the development of anti-cancer vaccines (5-7). Aberrant glycosylations include loss or over-expression of certain structures, the persistence of truncated structures and the emergence of novel structures. The structural differences were later supported by many histological evidences using lectin-staining compared with healthy and malignant tissue. (Turner G A (1992) Clin Chim Acta 208:149-171; Gabius H J (2000) Naturwissenschaften 87:108-121.)
More recently, tumor associated carbohydrate antigens were identified by monoclonal antibodies and mass spectrometry. (Shriver Z, et al. (2004) Nat Rev Drug Disc 3:863-873; Pacino G, et al. (1991) Br J Cancer 63:390-398.) To date, numerous tumor associated antigens expressed on cancer cells in the form of glycolipids or glycoproteins have been characterized and correlated to certain types of cancers. (Bertozzi C R, Dube D H (2005) Nat Rev Drug Discovery 4:477-488.) Although relatively little is known about the role of surface carbohydrates play in malignant cells, passively administered or vaccine induced antibodies against these antigens have correlated with improved prognosis.
Of the tumor associated glycans reported, the glycolipid antigen Globo H (Fucα1→2 Galβ1→3 GalNAcβ1→3 Galα1→4 Galβ1→4 Glc) was first isolated and identified in 1984 by Hakomori et al. from breast cancer MCF-7 cells. (Bremer E G, et al. (1984) J Biol Chem 259:14773-14777.) Further studies with anti-Globo H monoclonal antibodies showed that Globo H was present on many other cancers, including prostate, gastric, pancreatic, lung, ovarian and colon cancers and only minimal expression on luminal surface of normal secretory tissue which is not readily accessible to immune system. (Ragupathi G, et al. (1997) Angew Chem Int Ed 36:125-128.) In addition, it has been established that the serum of breast cancer patient contains high level of anti-Globo H antibody. (Gilewski T et al. (2001) Proc Natl Acad Sci USA 98:3270-3275; Huang C-Y, et al. (2006) Proc Natl Acad Sci USA 103:15-20; Wang C-C, et al. (2008) Proc Natl Acad Sci USA 105(33):11661-11666) and patients with Globo H-positive tumors showed a shorter survival in comparison to patients with Globo H-negative tumors. (Chang, Y-J, et al. (2007) Proc Natl Acad Sci USA 104(25):10299-10304.) These findings render Globo H, a hexasaccharide epitope, an attractive tumor marker and a feasible target for cancer vaccine development.
As an example, the Globo H-based glycoconjugate vaccines are currently undergoing large-scale clinical trials and have shown promise in therapeutic treatment (8, 9). Studies on the immune response to pathogenic microorganisms (such as Haemophilus influenza type B and Streptococcus pneumonia) have demonstrated that polysaccharides containing repeating antigenic units are generally T cell-independent (TI) (10, 11).
Globo H is a cancer antigen overly expressed in various epithelial cancers. It has been suggested that this antigen can serve as a target in cancer immunotherapy. While vaccines have been developed to elicit antibody responses against Globo H, their anti-cancer efficacies are unsatisfactory due to low antigenicity of Globo H.
In breast cancer, Globo H expression was observed in >60% of ductal, lobular, and tubular carcinoma, but not in nonepithelial breast tumors (Mariani-Constantini R et al., (1984) Am. J. Pathol. 115:47-56). Globo H is not expressed in normal tissue except for weak expression in the apical epithelial cells at lumen borders, a site that appears to be inaccessible to the immune system (Id.; Zhang S. et al., (1997) Int. J. Cancer 73:42-49).
Globo H also is expressed in breast cancer stem cells (BCSCs). Flow cytometry revealed Globo H is expressed in 25/41 breast cancer specimens (61.0%). Non-BCSCs from 25/25 and BCSCs from 8/40 (20%) express Globo H. The stage-specific embryonic antigen 3 (SSEA-3), the pentasaccharide precursor of Globo H, is expressed in 31/40 (77.5%) tumors. Non-BCSCs from 29/31 and BCSCs from 25/40 (62.5%) expressed SSEA-3. (Chang W-W. et al., (2008) Proc Natl Acad Sci USA 105(33):11667-11672.)
There is a need for a new vaccine capable of eliciting high levels of immune responses targeting Globo H and related antigens.